I'm back with 6.2 pounds of LP and the tank is as full as is possible.
Once again, bbq is off, no hand on the tank, wet probe end, & no lights on.
3 readings with 4 misfires/flashing.
Also used the hot water method which gave the precise level.
The device doesn't work once you get off of a flat surface (90 degrees) as you can see below where the tank tapers at the top.
Pictures below -